Method of forming a CMOS structure having gate insulation films of different thicknesses

ABSTRACT

The present invention is drawn to a semiconductor integrated circuit device employing on the same silicon substrate a plurality of kinds of MOS transistors different in magnitude of tunnel current flowing either between the source and gate or between the drain and gate thereof. These MOS transistors include tunnel-current increased MOS transistors at least one of which is for use in constituting a main circuit of the device. The plurality of kinds of MOS transistors also include tunnel-current reduced or depleted MOS transistors at least one of which is for use with a control circuit. This control circuit is inserted between the main circuit and at least one of two power supply units. The control circuit is responsive to receipt of a control signal supplied thereto for controlling the flow of a current either between the source and gate or between the drain and gate of the tunnel-current increased MOS transistor for use with the main circuit in such a way that the current flow is selectively permitted during certain time period and that it is inhibited during another period.

This is a divisional application of U.S. Ser. No. 09/155,801, filed Oct.6, 1998 now U.S. Pat. No. 6,307,236, which is a 371 of PCT/JP97/01191,filed Apr. 8, 1997.

TECHNICAL FIELD

The present invention relates generally to semiconductor integratedcircuit (IC) devices configured using metal oxide semiconductor (MOS)transistors, and more particularly to a semiconductor IC device whichemploys specific MOS transistors with a gate insulation film thin enoughto permit flow of tunnel current therein and which is adaptable for usewith low-power circuitry operable with low voltages of 2 volts or less.

BACKGROUND ART

One prior known semiconductor integrated circuit device employing highlyminiaturized MOS transistors fabricated by microelectronics fabricationtechnology is disclosed, for example, in a paper entitled “Limitation ofCMOS Supply-Voltage Scaling by MOSFET Threshold-Voltage Variation,” 1994Custom Integrated Circuit Conference (CICC), pp. 267-270. This paperalso teaches the correlation of the transistor threshold value versusflow of leakage current during standby periods.

DISCLOSURE OF THE INVENTION

Currently available standard MOS transistors are typically designed tooperate with a gate voltage of from 1.8 to 2.5 volts (so-called the“gate-to-source” voltage which is normally equivalent to the powersupply voltage) while making use of a gate insulation film ranging from5 to 6 nanometers (nm) in thickness. Generally, as the integrationdensity of MOS transistors increases, the transistor size decreases, andthe thickness of the gate insulation film decreases accordingly. Thepresent inventors Presently predict that MOS-IC devices of the nextgeneration will require use of further miniaturized MOS transistorsoperable with a gate voltage of 2 volts or less while reducing thethickness of gate insulation film down at 4 nm or less.

Principally, it may be considered that the operation speed of MOStransistors remains inversely proportional to the gate insulation filmthickness decreases—that is, as this thickness decreases, the MOStransistor speed increases. However, this does not come withoutaccompanying a “trade-off” penalty: When the MOS gate insulation filmbecomes too thinner, a tunnel current begins flowing therethrough. Thiscan result in an increase in leakage current (tunnel leakage current),such as a source-to-gate current or drain-to-gate current, whichinherently does never take place in standard MOS transistors. Suchincrease in tunnel leakage current in turn leads to an increase in powerdissipation of MOS transistors during standby periods thereof. In thedescription such dielectric films permitting tunnel current leakage willbe referred to as the “thin” gate insulation film; likewise, certain MOStransistors employing such dielectric film will be called the“thin-film” MOS transistors hereinafter. On the contrary, standard MOStransistors in which such tunnel leakage current does not flow will bereferred to as the “thick-film” MOS transistors. The “tunnel currentleakage” problem has been also discussed in the monthly journal titled“Semiconductor World,” July 1995 at pp. 80-94; unfortunately, this iscompletely silent about any ideas for solving this problem.

A mechanism of an increase in power dissipation during standby due totunnel current will be discussed more precisely in conjunction with thegraphs shown in FIG. 10.

See FIG. 10(a). This is a graphical representation showing experimentalresults concerning the drain voltage versus drain currentcharacteristics of one thick-film MOS transistor. Plotting experimentaldata in this graph assumes that its gate oxide film measuresapproximately 6 nm in thickness. Since the oxide film employed herein isthick enough to render negligible the tunnel leakage current which canflow between the gate and source or between the gate and drain.

See FIG. 10(b), which presents the drain-voltage/drain-currentcharacteristics of a thin-film MOS transistor. This assumes that a gateoxide film used is 3.5 nm in thickness. Since the oxide film is thin,leakage current can flow between the gate and source and also betweenthe gate and drain thereof. Accordingly, even where the drain voltage isat zero volts, a non-negligible amount of current flows between the gateand drain when its gate voltage is not zero volts. In the graph of FIG.10(b), a drain current of 0.5 milliamperes (mA) or more or less wasderived when the gate voltage is 2.0 volts.

In complementary MOS (CMOS) circuitry configured using thick-film MOStransistors, since gate leakage current remains negligible in amount,any constant current (DC current) will by no means flow insofar asleakage current is absent between the source and drain. On the contrary,with CMOS circuitry employing thin-film MOS transistors, gate leakagecurrent does flow so that constant current (DC current) flowsaccordingly. This means that some power dissipation arises even wherethe circuitry is inoperative.

See FIG. 11, which shows the relation of the thickness of gateinsulation film versus gate leakage current. Even when the gate voltageis at 2 to 3 volts or around it, if the gate insulation film is 6 nm orgreater in thickness, then any resultant tunnel current remains harmlessin practical applications. On the other hand, it may be seen by thoseskilled in the art that even if the gate voltage is potentiallydecreased to range from 2 to 1.5 volts which may be lower than ever, theleakage current will no longer remain negligible in magnitude once afterthe thickness of gate insulation film is reduced at approximately 3 nm.Presumably, if the gate voltage is 2 volts or more or less, then theboundary exists at a 4-nm range of gate insulation film thickness oraround it. According to the teachings of the Semiconductor Worlddocument, it has been pointed out that the tunnel effect in quantumtheory takes place with a 5-nm gate insulation film point being as thecriticality. This document also teaches that a remarkable tunnel currentcan occur not only when the gate insulation film is as thin as 1.5 nmbut also when it falls within a range of from 3 to 3.5 nm. As can beseen from the graph of FIG. 11, while the gate voltage tends to belowered for reduction in power dissipation; even in this situation, whenthe gate insulation film becomes thinner to decrease from 2.9 to 2.0 nmin thickness, large leakage current begins flowing even upon applicationof a gate voltage of 1 volt or below. Additionally, it is currentlypresumed that a minimal thickness of gate insulation films capable ofretaining the nature of silicon oxide is about 10 angstroms.

Another approach is known which suppresses a sub-thresholdsource-to-drain leakage current by potentially raising the thresholdvalue of MOS transistors. However, even with use of such approach, itstays impossible in principle to reduce standby power dissipation due tothe flow of source-to-gate tunnel current.

While the gate leakage current (tunnel current) might be under controlby increasing the thickness of gate insulation films to reduce standbypower dissipation involved, this does not come without accompanying apenalty: As discussed supra, if such MOS transistors are employed forcircuitry then operation speed decreases making it impossible or atleast greatly difficult to attain any desired performance.

It is therefore an object of the present invention to provide asemiconductor integrated circuit device capable of reducing standbypower dissipation without having to degrading circuit operation speed.

In order to attain the foregoing object, the invention provides alow-power/high-performance semiconductor integrated circuit device byselective use of different kinds of MOS transistors including thick-filmMOS transistors and thin-film MOS transistors, wherein the former isnegligible in flow of tunnel leakage current whereas the latter iscapable of operating at high speeds while accompanying the tunnelcurrent leakage problem.

In accordance with the principles of the invention, there is provided asemiconductor integrated circuit device including on the same substratea plurality of kinds of MOS transistors different in magnitude of aleakage current flowing either between the source and gate or betweenthe drain and gate. The semiconductor integrated circuit device isconfigured to have main circuitry constituted from at least one MOStransistor of the plurality of kinds of MOS transistors being greater inleakage current, and control circuitry inserted between the maincircuitry and at least one of two power supplies and comprised of atleast one MOS transistor less in leakage current.

It should be noted that intended high-speed characteristics may besuccessfully accomplished by designing the MOS transistors such that thegate insulation film measures 3.5 nm or less in thickness: Also,rendering it thinner at 3.0 nm and further thinner at 2.0 nm or less mayenable the operation speed to further increase. However, as theoperation speed increases, tunnel leakage current will likewise increasein magnitude. In view of this, it may be desirable that the MOStransistors of reduced leakage current be specifically employed forinterruption or interception of a standby voltage(s) as applied to thethin-film MOS transistors. Intended advantages may be sufficientlyattained whenever the power-supply intercept MOS transistors measure 5.0nm or greater in thickness; if extra high speed requirements are notrequired when reduction to practice, it may be permissible for them tomeasure 10.0 nm or greater.

These MOS transistors may be structured to offer any desiredcharacteristics by suitably designing the thickness of gate insulationfilm or changing either carrier density or distribution at the gateelectrode, drain and/or source electrode. Generally, increasing the gateinsulation film thickness requires that the gate length increase invalue accordingly.

In regard to the microelectronics fabrication process, thecharacteristic control or adjustment may become accurate when the twokinds of—namely, thin-film and thick-film—MOS transistors aremanufactured such that the gate insulation films and gate electrodesthereof are formed at separate process steps. Especially, it will berecommendable that thick gate insulation films be formed prior toformation of thin gate insulation films because of the fact that thelatter is difficult than the former in control of process parametersduring fabrication. In addition, in cases where such two kinds of MOStransistors are formed separately, forming a protective dielectric filmon resultant gate electrode layer may enable suppression or eliminationof occurrence of gate-electrode degradation otherwise occurring due toexecution of succeeding processes.

It should be noted here that in the semiconductor integrated circuitdevice in accordance with the instant invention, the thin-film MOStransistors are preferably selected for use with specific circuitryparts under strict requirements of high-speed characteristics,including, but not limited to, information signal processor circuits,such as logic function units (logic circuits such as NAND gates, NORgates and the like) as built in central-processing units (CPUs), latchcircuits, high-speed memory cell arrays, and others.

In contrast, switch elements for interruption of power supply duringstandby periods of these thin-film MOS transistors may be configuredusing thick-film MOS transistors, which function as the power-supplyintersect transistors. Also, any circuitry parts without high-speedrequirements as well as circuits under strict requirements of highvoltage withstand characteristics may be configured by such thick-filmMOS transistors. Memory cells with no high-speed requirements such asstatic random access memory (SRAM), dynamic RAM (DRAM), mask read-onlymemory (mask ROM) are one example. Protective circuitry as inserted forprevention of gate-insulation film dielectric breakdown is anotherexample. Preferably, those of the thick-film MOS transistors which areto be applied with high voltages come with a specifically designedsource/drain structure—that is, electric field relaxation structureincluding, but not limited to, a lightly-doped drain (LDD) structure.

It should be also noted that in cases where the semiconductor integratedcircuit device of the invention is arranged as an IC chip, it will berecommendable that a level converter circuit for potential levelconversion of electrical signals be built therein in order to “absorb”any possible differences in potential level between incoming signals tothe chip and outgoing ones from it. When this is done, it is desirablein view of reliability that thick-film MOS transistors be employed incertain circuit part for receiving high potential external signalswhereas thin-film MOS transistors be in remaining circuit part forhandling relatively low potential internal or “in-chip” signals.

The memory cells configured using thick-film MOS transistors mayfunctionally include at least one of register files, cash memories,translation look-aside buffers (TLBs), and DRAM cells; if this is thecase, it is preferable that the memory cells are arranged to storetherein data during standby periods. The invention however should not belimited exclusively thereto and may alternatively be modified such thatthese include first kinds of memory cells of high access rate and secondkind of ones lower in access rate than the former, wherein the MOStransistors constituting the first memory cells are greater in leakagecurrent than those forming the second memory cells.

It should further be noted that upon interruption of the power supply ofthe thin-film MOS transistors by the power supply intercepttransistor(s), it is possible by providing a level hold circuit—this maybe configured using thin-film MOS transistors for retaining or holdingthe last potential level of an output of logic circuit operativelyassociated therewith—to eliminate any adverse influence or affection bypower supply intercept of the thin-film MOS transistors. Preferably,such level hold circuit may be formed of one or more thick-film MOStransistors less in leakage current magnitude.

The thin-film MOS transistors as employed in accordance with theprinciples of the invention may advantageously serve to reduce powerdissipation significantly by interrupting or “intercepting” power feedduring standby periods in light of the fact that leakage current canincrease in magnitude even where these thin-film MOS transistors aredesigned to operate with extra low gate voltage below 2 volts, such as0.8 volts or 1.2 volts or therearound, by way of example.

Preferably, that the leakage current-increased MOS transistors andleakage current-decreased ones are potentially driven by use ofdifferent gate voltages therefor. Practically, the leakagecurrent-increased MOS transistors are to be driven upon application ofcertain voltage between the gate and source of each one, which voltageis lower than that being applied to the leakage current decreased MOStransistors.

In accordance with one aspect of the invention, a semiconductorintegrated circuit device is provided which includes first and secondMOS transistors as formed on the same silicon chip substrate. Arespective one of the first MOS transistors has an insulative film of4-nm thick or less as laid between the source and drain thereof orbetween its drain and gate; a corresponding insulative film of eachsecond MOS transistor measures more than 4 nm in thickness.

In accordance with another aspect of the invention, a semiconductorintegrated circuit device includes first and second MOS transistors asformed on the same silicon chip substrate. A respective one of the firstMOS transistors has an insulative film of 4-nm thick or less as laidbetween the source and drain thereof or between its drain and gate; eachsecond MOS transistor has an insulative film between the source and gateor between the drain and gate thereof, which film is greater inthickness than the first MOS transistors. The second MOS transistors arespecifically adaptable for use in controlling flow of source-to-gatecurrent or drain-to-gate current of the first MOS transistors.

In accordance with still another aspect of the invention, asemiconductor integrated circuit device includes first and second MOStransistors as formed on the same silicon chip substrate. A respectiveone of the first MOS transistors has an insulative film of 4-nm thick orless as laid between the source and drain or between the drain and gatethereof. The second MOS transistors are adaptable for use ininterrupting transfer of associated power supply voltages toward thefirst MOS transistors. The semiconductor integrated circuit devicefurther includes a level hold circuit for holding the last potentiallevel of an output signal of each first MOS transistor duringinterruption of power supply.

In accordance with yet another aspect of the invention, a semiconductorintegrated circuit device includes first and second MOS transistors asformed on the same silicon chip substrate. The first MOS transistors areinherently greater in magnitude of leakage current flowing between thesource and gate or between the drain and gate thereof whereas the secondMOS transistors remain less in leakage current than the first ones. Theintegrated circuit device is specifically arranged so that the secondMOS transistors are driven by a predefined high voltage which is higherthan that being applied to the first MOS transistors.

In accordance with a further aspect of the invention, a semiconductorintegrated circuit device responsive to receipt o: an input signalhaving a specified amplitude voltage Vcc2 is arranged to include a levelconverter circuit for generating and issuing an in-chip signal bypotentially reducing the amplitude voltage of an input signal, whereinleakage current occurrable between the gate and source or between thegate and drain of a MOS transistor accepting the in-chip signal isgreater than that in another MOS transistor receiving the input signal.

When practicing the invention by applying it to integrated circuitdevices such as those for use with microcomputers, the semiconductorintegrated circuit device comes with an arithmetic processor unit and adata storage unit as configured using MOS transistors, which circuit mayinclude at least one of the mask ROM, SRAM, and DRAM. The MOStransistors constituting one or more logic circuits in the arithmeticProcessor has a gate insulation film which is less in thickness thanthose forming memory cells of the storage unit.

In accordance with a still further aspect of the invention, asemiconductor integrated circuit device includes on the same siliconsubstrate a plurality of kinds of MOS transistors including first MOStransistors and second MOS transistors different from each other inmagnitude of tunnel current flowing between the source and gate orbetween the drain and gate. The semiconductor integrated circuit devicealso includes a main circuit configured using at least one tunnelcurrent increased MOS transistor. The device further includes acontroller circuit as operatively is coupled to the main circuit andalso to at least one of two power supply units. This controller employsat least one of tunnel current decreased (or absent) MOS transistor. Thecontroller is responsive to a control signal fed thereto for providingcontrol so as to selectively permit and inhibit the flow of a currentbetween the source and gate or between the drain and gate of the tunnelcurrent increased MOS transistor for use in constituting the maincircuit.

One characterizing feature of the semiconductor integrated circuitdevice lies in that the plurality of kinds of MOS transistors includeMOS transistors different in gate insulation film thickness, oralternatively MOS transistors of the same conductivity type having gateelectrodes doped with the same kind of impurity to different degrees ofdopant concentration.

Another feature is that where MOS transistors different in gateinsulation film thickness are employed, those MOS transistors eachhaving a thick gate insulation film is provided with a side wall spacerwhich is adhered coating the side wall of its gate electrode. The spacermay be made of a chosen insulative material chemically insensitive tohydrofluoric acid. This side wall spacer may be for use as a mask forfabrication of the LDD structure, supra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing, in cross-section, some of the major stepsof the manufacture of a semiconductor integrated circuit device inaccordance with one preferred embodiment of the present invention.

FIG. 2 illustrates in cross-section some of the major steps of themanufacture of a semiconductor integrated circuit device in accordancewith another embodiment of the invention.

FIG. 3 is a plan view of a semiconductor integrated circuit deviceembodying the invention.

FIG. 4 depicts a cross-sectional view of the device as taken along lineA—A′ of FIG. 3.

FIG. 5 is a circuit diagram showing an equivalent circuit configurationof a semiconductor integrated circuit device embodying the invention.

FIG. 6 shows a circuit configuration of another embodiment.

FIG. 7 shows a circuit configuration of a further embodiment.

FIG. 8 shows a circuit configuration of a still further embodiment.

FIG. 9 shows a circuit configuration of yet another embodiment.

FIG. 10 presents graphs representative of typical currentcharacteristics of one standard MOS transistor and of a tunneling MOStransistor permitting flow of tunnel current through its gate insulationfilm.

FIG. 11 is a graph representative of the relation of gate insulationfilm thickness and gate current density.

FIG. 12 is a circuit diagram showing a configuration of a furtherembodiment of the invention.

FIG. 13 shows a circuit configuration of a further embodiment of theinvention.

FIG. 14 is a circuit configuration of a further embodiment of theinvention.

FIG. 15 is a circuit configuration of a further embodiment of theinvention.

FIG. 16 is a circuit configuration of a further embodiment of theinvention.

FIG. 17 is a circuit configuration of a further embodiment of theinvention.

FIG. 18 is a circuit configuration of a further embodiment of theinvention.

FIG. 19 is a circuit configuration of a further embodiment of theinvention.

FIG. 20 is a circuit configuration of a further embodiment of theinvention.

FIG. 21 is a circuit configuration of a further embodiment of theinvention.

FIG. 22 is a plan view of an integrated circuit chip embodying theinvention.

FIG. 23 is a plan view of an integrated circuit chip also embodying theinvention.

FIG. 24 is a plan view of an integrated circuit chip embodying theinvention.

FIG. 25 is a plan view of an integrated circuit chip embodying theinvention.

FIG. 26 is a plan view of an integrated circuit chip embodying theinvention.

FIG. 27 is a circuit diagram of a potential reduction circuit inaccordance with an embodiment of the invention.

FIG. 28 is a circuit diagram of a potential reduction circuit inaccordance with a further embodiment of the invention.

FIG. 29 is a circuit diagram of a potential reduction circuit alsoembodying the invention.

FIG. 30 is a circuit diagram of an input/output circuit.

FIG. 31 is a circuit diagram of a level converter circuit with levelhold functions.

FIG. 32 is a circuit diagram of another level-holdable level converter.

FIG. 33 is a circuit diagram of a standby controller circuit.

FIG. 34 is a block diagram of a microcomputer system embodying theinvention.

FIG. 35 is a cross-sectional view of an input/output circuit.

FIG. 36 is a circuit diagram of a mask ROM embodying the invention.

FIG. 37 shows another configuration of a mask ROM also embodying theinvention.

FIG. 38 is a partial cross-sectional view of the mask ROM shown in FIG.37.

FIG. 39 is a circuit diagram of a further mask ROM.

FIG. 40 illustrates a partial cross-section of the mask ROM of FIG. 39.

FIG. 41 is a circuit diagram of a further mask ROM.

FIG. 42 illustrates a partial cross-section of the mask ROM of FIG. 40.

FIG. 43 depicts a circuit configuration of a DRAM embodying theinvention.

FIG. 44 is a circuit diagram of a sense amplifier as employed in theDRAM of FIG. 43.

FIG. 45 shows a circuit configuration of a sense-amplifier drive signalgenerator circuit as used in the FIG. 43 DRAM.

FIG. 46 shows a circuit configuration of a main amplifier used in theFIG. 43 DRAM.

FIG. 47 is a circuit diagram of an SRAM embodying the invention.

FIG. 48 is a circuit diagram showing a word decoder, word driver, andlevel converter as employed in the device of FIG. 47.

FIG. 49 is a circuit diagram showing a sense amplifier and itsassociative write circuit used in the FIG. 47 device.

FIG. 50 illustrates, in enlarged cross-section, major steps of thefabrication of an n-type MOS transistor structure embodying theinvention.

BEST MODE EMBODYING THE INVENTION

Several embodiments practicing the invention as will be describedhereafter are principally featured in that tunnel current occurrable MOStransistors permitting the flow of a tunnel current either between thedrain and gate of each transistor or between the drain and gate thereofare provided on a single chip substrate along with tunnel current absent(or much decreased) MOS transistors substantially preventing such tunnelcurrent flow between them, and that the tunnel current available MOStransistors are adaptable for use in constituting a main circuit withlogic gates whereas the tunnel current depleted (or much reduced) MOStransistors are in forming a controller circuit which controls selectionof operation modes between transferring power supply voltages toward themain circuit and interrupting or “intercepting” the power supplythereto. With such an arrangement, it is possible to reduce powerdissipation during standby periods of the MOS transistors constitutingthe main circuit without having to affecting the circuit operation speedthereof.

Several semiconductor integrated circuit devices embodying theprinciples of the instant invention will be described with reference tothe accompanying figures of the drawing.

FIGS. 1 and 2 are each a pictorial representation for explanation of amanufacturing procedure of a semiconductor integrated circuit devicethat satisfies the 0.18-micrometer rules in highly advancedmicroelectronics fabrication technology, which device has both tunnelcurrent available MOS transistors and tunnel current depleted (or muchreduced) MOS transistors on the same silicon substrate. FIG. 1 shows afirst embodiment of the invention employing thickness differentinsulative films whereas FIG. 2 illustrates a second embodimentutilizing the impurity concentration differing scheme. Note that theminimal fabrication dimension as used herein is normally definable bythe resulting gate length of MOS transistors; it is hardly affected byoverlaps of a gate electrode with its associated source and drain. Onthe other hand, another manufacturing method is known for intentionallyshortening a gate length resulting from evaluation of electricalproperties rather than the physical fabrication dimension regarding thegate length. If this is the case, the gate length based on suchelectrical properties—say, the “effective gate length”—becomes a keyparameter. In FIGS. 1 and 2, one form is shown which has no essentialdifferences between the physical gate length reflecting fabricationdimensions and the effective gate length; however, it should be notedthat the present invention may also remain applicable to themanufacturing method for intentionally shortening the effective gatelength so that it is less than the physical gate length.

The first embodiment will now be explained in conjunction with FIG. 1.This embodiment is drawn to the manufacture of an exemplary IC deviceattaining copresence of a tunnel current available MOS transistor and atunnel current depleted MOS transistor without any substantive flow oftunnel current therein on the same chip substrate by differing thethickness of gate insulation film between them. While this embodiment isintended to fabricate such two different kinds of gate insulation filmsby use of both p-type MOS transistors and n-type MOS transistors,cross-sections presented in FIG. 1 assume use of p-type MOS transistorsonly for purposes of reducing complexity of illustration. An n-typesilicon substrate 101 is prepared having a top surface on which anelement isolation dielectric region 102 made of a thermal oxide film of300 nm thick is formed. In the substrate surface, n-type impurity-dopedlayers 103 and 104 are formed. These impurity layers 103, 104 are 1×10¹⁷atoms cm⁻³ in average concentration and are inherently necessary forelement isolation from n-type MOS transistors (both thin-film andthick-film ones) copresent within a single chip. Any one of currentlyavailable impurity doping techniques may be employed to form n-typelayers 103, 104.

After an impurity is doped by ion implantation techniques causing eachMOS transistor to exhibit a predefined threshold value, a siliconthermal oxide film 105 is formed to a predetermined thickness, forexample, 10 nm, on the entire surface of the substrate structure;subsequently, a polycrystalline silicon or “polysilicon” layer 106 isdeposited overlying the thermal oxide film 105. Layer 106 is thenentirely doped with an impurity of a chosen material such as phosphoruscausing resultant polysilicon layer 106 to be adjusted at 1×10²⁰ cm⁻³ inaverage concentration of phosphorus impurity. The phosphorus ionintroduction may be performed by ion implantation to 2×10¹⁵ atoms cm⁻²at acceleration energy of 40 kiloelectronvolts (KeV); alternatively, thesame may be attained by use of a boron-doped polysilicon or usingpredeposition techniques. Subsequently, a silicon oxide film isdeposited as a gate electrode protective film 107 to a thickness of 50nm on the entire surface of resultant structure at step (a) of FIG. 1.The function of this layer 107 will be described later.

The thermal oxide film 105, polysilicon layer 106 and gate electrodeprotective layer 107 are then subject to patterning usingphotolithography and dry etching techniques forming an insulated“island” 108 on substrate 101. This island 108 measures 0.18 micrometers(μm) or greater in gate length fabricated. Island 108 will constitutethe gate electrode of a thick-film MOS transistor. Then, boron fluorideis introduced into the substrate structure, with the gate electrodebeing as a mask therefor, to a concentration of 2×10¹⁴ cm⁻² atacceleration voltage of 20 KeV, forming spaced-apart p-type layers 109Lin impurity region 104 of substrate 101. These layers 109L may act asthe source and drain regions of the lightly-doped drain (LDD) type. Thisis done because a voltage being applied to such thick-film MOStransistor is not potentially low enough to ensure that it remains freefrom any possible adverse influence of hot carriers which in turn servesto degrade the characteristics thereof. To retain reliability, it isrecommendable to employ electric field relaxation structures includingthe LDD structure. Introduction of p-type impurity makes use of ionimplantation techniques with average concentration being set at 5×10¹⁸cm⁻³ although this value is adjustable in conformity with thecharacteristics of MOS transistors required.

Dielectric films 110 are formed on the opposite side walls of the gateelectrode 108. These side wall spacers 110 may be made of siliconnitride of 100 nm thick. Spacers 110 will act as protectors whichprotect the silicon oxide film beneath gate electrode 108 (this oxidefilm functions as the gate insulation film of gate 108) against unwantederosion due to physical and/or chemical reaction of hydrofluoric acidsolution as employed during a later washing process of the overallsurface of resultant substrate structure. Then, boron fluoride isintroduced to a concentration of approximately 2×10¹⁵ cm⁻² atacceleration voltage of 20 KeV, thereby forming spaced-apart p-typelayers 109 in region 104 of substrate 101, which layers will act as thesource and drain regions with respect to the gate electrode 108 of theLDD MOS transistor. The doping of a p-type impurity is done by ionimplantation techniques to an average concentration of 5×10¹⁹ cm⁻³ ormore or less.

The resulting gate insulation film of the MOS transistor with gateelectrode 108 thus formed may measure 10 nm in thickness by way ofexample. In cases where MOS transistors are designed satisfying the0.18-μm gate length criteria shown herein, the power supply voltagefalls within a range of from 1.8 to 1.5 volts. An electric field ascreated within the gate oxide film will be 1.8 megavolts per squarecentimeter (MVcm⁻²) or less. In this situation a tunnel current is verylimited in magnitude to be as low as 10⁻²⁰ amperes per square centimeter(Acm⁻²), which will by no means affect or disturb execution of normalMOS transistor operations. Any increase in power dissipation will nolonger occur due to gate leakage current. This p-type MOS transistorhardly permits flow of a tunnel current since its gate insulation filmis thick (10 nm in this embodiment). This p-type MOS transistor is foruse in transferring (during ON periods) a packet of charge carriers fromthe power supply to a main circuit and in interrupting (during OFFperiods) the same.

Next, after formation of the side wall spacers 110, resultant substratestructure is washed using hydrofluoric acid solution. The structure isthen subject to anodization selectively forming a thermal oxide film 111to a thickness of 3.5 nm on the entire exposed top surface of thesilicon substrate 101 other than certain surface areas in whichelement-separation films 102 and gate electrode 108 are formed. Then, apolysilicon layer 112 is deposited on the entire surface of resultantstructure to a thickness of 180 nm, which layer 112 overlieselement-separators 102 and gate 108 at step (b) of FIG. 1. Polysiliconlayer 112 is entirely doped with a phosphorus ions to a concentration of5×10¹⁵ cm⁻² at acceleration voltage of 25 KeV allowing layer 112 to beof n-type conductivity with the average impurity concentration of 1×10²⁰cm⁻³. Thereafter, a silicon oxide film 113 is formed thereon to athickness of 100 nm, by way of example, at step (b). Film 112 is as aprotective film of gate 108.

Then, the thermal oxide film 111, polysilicon 112 and silicon oxide 113are subject to patterning by photolithography and dry etching techniquesthus providing another insulated gate electrode 114 of the 0.18 μm gatelength. This gate electrode 114 constitutes the gate of a thin-film MOStransistor. Note here that unless otherwise the protective film 107 isformed at the previous step were absent, the gate electrode 106 of thethick-film MOS transistor would have result in being removed away duringformation of oxide 113. In this sense the layer 107 is inevitable. Justafter the gate formation step, boron fluoride is introduced by ionimplantation into layer 103 to a concentration of 2×10¹⁵ cm⁻² at 20 KeVproviding p-type source and drain regions 115 at step (c) of FIG. 1.Since a voltage applicable to thin-film MOS transistors will beextremely lower in potential, the aforementioned “hot-carrier influence”problem encountered with thick-film MOS transistors is not so seriousavoiding the need of employing any electric field relaxation structure,such as the LDD structure discussed supra.

Subsequently, further ion implantation is done for suppression of theshort-channel effect although this step is not depicted in the drawingfor elimination of complexity of illustration only. After formation ofan interlayer dielectric film 116 overlying the entire surface ofresultant structure, a patterned first metal chip-lead layer 117 isformed on film 116 at a first level over the surface of substrate 101,providing electrical interconnection with respective terminal pads ofMOS transistors at step (d). When necessary, second and third patteredchip-lead layers may be additionally formed at second and third levelsover substrate 101. A resultant MOS transistor with gate 114 will becalled the “thin-film” MOS transistor because of the fact that thiscomes with a reduced-thickness or “thin” gate insulation film beneathgate 114. This thin-film MOS transistor is such that even uponapplication of low power supply voltage of 1.8 volts, an electric fieldcreated within its gate insulation film will be 5 MVcm⁻² or greaterwhile simultaneously causing it to measure 1×10⁻⁶ Acm⁻² in magnitude ofgate leakage current. As can be readily seen by those skilled in theart, this thin-film MOS transistor was manufactured satisfying thecurrently available submicron-scaling rules in the semiconductormicrofabrication technology; in this regard, this MOS transistor remainsadaptable for use with the on-chip main circuitry. Additionally, it isdesirable that the thick-film MOS transistors be greater than thin-filmMOS transistors in gate length—i.e. a minimal gate length of thosetransistors on substrate 101. The threshold value of thick-film MOStransistors is required to be greater than that of then-film ones; onthe contrary, it has been well known that if the gate oxide filmthickness alone is simply increased then resultant threshold value willdecrease accordingly. The lower the threshold value, the easier the MOStransistors fail to completely turn off; in such situation a currentmight attempt to flow in thin-film transistors disenablingaccomplishment of intended advantages of the present invention. Thisphenomenon may be avoided by increasing the source-to-draindistance—namely, by enlarging the gate length. This method well matchesthe MOS transistor design scheme which has been generally called the“scaling rule.” More specifically, employing those MOS transistorswithout the scaling treatment will be sufficient; however, this does notcome without certain “pain” in that resultant chip surface areaincreases.

Another approach is available to increase the impurity concentration ofthe channel section of thick-film MOS transistors. One advantage of thisapproach is that on-chip MOS transistors are capable of decreasing inoccupation area on the substrate due to the capability of shortening thegate length as compared with other approaches; a disadvantage is thatthe MOS transistor withstand voltage rating and reliability are reduceddue to an increase of the internal electric field within each MOStransistor beyond an expected level as defined by the scaling rules.

FIG. 50 is one modification employing n-type MOS transistors only. Thisassumes that these transistors are fabricated on the same substratealong with. those transistors discussed previously in connection withFIG. 1 in a way as will be set forth below.

An n-type silicon substrate 5101 is prepared with an element isolationdielectric region 5102 made of a thermal oxide film of 300 nm thickbeing formed thereon, and n-type impurity-doped layers 5103, 5104 asformed in the surface thereof. These layers 5103, 5104 are doped with animpurity to an average concentration of approximately 1×10¹⁷ cm⁻³, andare inherently necessary for element isolation with respect to“associate” p-type MOS transistors on the same chip substrate (includingboth thin-film and thick-film ones). The impurity introduction may beperformed using any one of currently available techniques.

After ion implantation is done causing respective MOS transistors toexhibit desired threshold values, a silicon anodization (thermal oxide)film 5105 is formed on resultant substrate structure to a predeterminedthickness of 10 nm, for example. Then, a polysilicon layer 5106 isdeposited thereon to a thickness of 120 nm. The resulting structure isnext doped with boron ions at its entire exposed surface causing theboron's average concentration to be adjusted to 1×10²⁰ cm⁻³ or abovewithin a polysilicon layer 5106 overlying thermal oxide 5105. The boronions may be doped by ion implantation to a concentration of 2×10¹⁵ cm⁻²at acceleration voltage of 40 KeV; alternatively, the same may beattained by use of previously boron-doped polysilicon. Subsequently, asilicon oxide film 5107 is formed as a gate electrode protection film onthe entire top surface of the structure to a thickness of 50 nm at step(a).

The thermal oxide film 5105, polysilicon layer 5106 and gate electrodeprotection layer 5107 are patterned by photolithography and dry-etchingtechniques forming on substrate 5101 a patterned gate electrode “island”5108, which has its fabrication gate length of 0.18 μm in view of theshort-channel effect. This island 5108 will constitute the gateelectrode of a thick-film MOS transistor. With this gate electrode as amask, arsenic is introduced to a concentration of 2×10¹⁴ cm⁻² at 35 KeVto thereby provide spaced-apart n-type impurity-doped layers 5109L inone n-type layer 5104 of substrate 5101. These layers 5109L act as thesource and drain regions of the LDD type with respect to gate electrode5108. The reason for this is as has been discussed in conjunction withFIG. 1. Introduction of n-type impurity herein may be done by ionimplantation techniques with the average concentration being set at5×10¹⁸ cm⁻³ although such value remain freely adjustable in conformitywith exact MOS transistor characteristics required.

Then, dielectric spacer layers 5110 are formed on the opposite sidewalls of the gate electrode 5108. These side-wall spacers 5110 may bemade of silicon nitride of 100-nm thick. Each side-wall spacer 5110 willfunction as a protective film that protects the silicon oxide filmbeneath the gate electrode 108—namely, a silicon oxide film underlyinggate 108 and serving as the gate insulation film of gate 5108—againstany possible erosion due to physical and/or chemical reaction of washingsolvent employed, such as hydrofluoric acid, during washing of theentire surface of resultant structure. The structure is doped withphosphorus impurity to a concentration of 2×10¹⁵ cm⁻² at 40 KeV forminga pair of n-type regions 5109 in layer 5104 in such a manner that eachis in contact with a corresponding one of region 5109L. These act as thesource and drain with respect to gate electrode 5108. The n-typeimpurity may be done by ion implantation techniques to averageconcentration of 5×10¹⁹ cm⁻³.

In this embodiment the gate insulation film of resultant MOS transistorhaving the gate electrode 5108 measures 10, by way of example.

After formation of the side-wall spacers 5110, the overall top surfaceof the structure is washed using hydrofluoric acid. A thermal oxide film5111 is formed to a thickness of 3.5 nm on selected area of the exposedsurface of the silicon substrate 5101 excluding certain areas offormation of the element separation dielectric region 5102 and gateelectrode 5108. Next, a polysilicon 5112 of 180-nm thick is entirelydeposited thereon. The resulting structure is doped with boron ions to5×10¹⁵ cm⁻² at acceleration voltage of 40 KeV, thereby providing ann-type polysilicon of 1×10²⁰ cm⁻³ in average impurity concentration. Asilicon oxide 5113 is then formed directly overlying n-type polysilicon5112 at step (b).

The thermal oxide film 5111, polysilicon 5112 and silicon oxide 5113 arethen patterned using photolithography and dry-etching techniques,defining another island 5114 of 0.18-μm gate length. Island 5114constitutes the gate electrode of a thin-film MOS transistor onsubstrate 5101. Resultant structure is next doped by ion implantationwith an impurity of arsenic to 2×10¹⁵ cm⁻² at 40 KeV providing in layer5103 spaced-apart n-type regions as the source and drain of thethin-film MOS transistor at step (c).

Subsequently, further ion implantation is carried out for suppression ofthe short-channel effect although such step is not shown in the drawingfor purposes of illustration only. After an interlayer dielectric film5116 is formed on the entire surface of resultant structure, a firstpatterned metal chip-lead layer 5117 is formed thereon for electricalinterconnection with respective transistor terminal pads at step (d). Ifnecessary second and third chip-lead layers may be formed insulativelyoverlying chip-lead metal pattern 5117.

A second embodiment is shown in FIG. 2, which is directed to oneexemplary manufacturing method of an IC chip having on the samesubstrate different Winds of MOS transistors: tunnel-current availableMOS transistors permitting flow of a tunnel current therein resultingfrom a change of impurity concentration at gate and source portions, andtunnel-current absent MOS transistors permitting no substantive tunnelcurrent flow therein. In this embodiment the cross-sections of p-typeMOS transistors alone are illustrated in FIG. 2 in the same manner asthat in the previous embodiment (FIG. 1). An n-type silicon substrate201 is prepared on which an element isolation dielectric region 202, ann-type impurity layer 203 and an n-type impurity layer 204 are formed asshown. Here, the n-type layer 203 is devoted to provision of a wellregion of transistors for use in constituting a main circuit whereasn-type layer 204 is for a well of power-supply intercept MOS transistorswhich selectively permit power feed to the main circuit during firstperiods and to interrupt it during second periods. Layers 203, 204 are1×10¹⁷ cm⁻³ or more or less in average impurity concentration. Impuritydoping into layers 203, 204 may be attained by any presently availabletechniques. After certain impurity is doped by ion implantation intoselected surface regions of transistor formation for adjustment ofthreshold values thereof, a thermal oxide film 205 is formed on theentire exposed top surface of substrate 210 to a thickness of 3.5 nm.Then, a polysilicon layer 206 of 180 nm thick is entirely depositedthereon at step (a).

A dose of phosphorus ions 207 a is introduced into certain substratesurface areas assigned to formation of standard transistors exhibitingnormal circuit operations, to a concentration of 2×10¹⁵ cm⁻² atacceleration voltage of 25 KeV providing an n-type polysilicon 207 abovelayer 203 of substrate 201.

Likewise, a dose of phosphorus ions 208 a is doped into differentsubstrate surface areas exclusively assigned to formation ofpower-supply intercept transistors exhibiting normal circuit operations,to a concentration of 2×10¹⁵ cm at 35 KeV forming an n-type polysilicon208 at step (b).

Due to a difference between process parameter settings at the phosphorusion introduction steps, resultant gate electrode of a power-supplyintercept transistor is “variable” in impurity concentration dependingupon locations in such a manner that the impurity concentrationincreases (up to approx. 1×10²⁰ cm⁻³) only at its upper sections whilecausing its lower section near the underlying gate insulation film tolocally decrease in impurity concentration (approx. 1×10¹⁷ cm⁻³).Accordingly, the gate-electrode lower section decreases in carrierdensity allowing the transistor to resemble in electrical characteristicthose MOS transistors with a thick gate insulation film. This may inturn minimize flow of a tunnel current via the gate insulation film.

After implantation of phosphorus ions 207 a, 208 a, a silicon oxide film209 of 100-nm thick is deposited on the entire surface of resultantsubstrate structure. Then, the thermal oxide film 205, n-typepolysilicon 208 and silicon oxide film 209 are subject to patterningprocess by photolithography and dry-etching techniques forming gateelectrode islands 210, 211 at step (c). Gate electrode 210 is 0.18 μm ingate length. Since gate electrode 211 is seen to have a thick gateinsulation film, its gate length is set at 0.18 μm or greater in view ofthe short-channel effect. After formation of gates 210, 211, a pair ofspaced-apart p-type regions 212 are formed in layer 203 in such a waythat these are essentially self-aligned with the overlying gate 210 thusproviding the source and drain regions of a MOS transistor. Similarly,spaced-apart p-type regions 213 are formed, as the transistor source anddrain, in the neighboring layer 204 to be self-aligned with itsoverlying gate 211 associated therewith at step (c). Ion implantationmay be employed for introducing of p-type impurity such that boronfluoride is introduced to 2×10¹⁵ cm⁻² at 20 KeV. In this embodimentalso, extra ion implantation for suppression of the short-channel effectis not specifically shown in the drawing for depiction complexityelimination purposes only. After formation of an interlayer dielectricfilm 214, a first metal chip-lead pattern 215 is formed thereon forelectrical interconnection with respective transistor terminal pads. Asnecessary, second and third chip-lead patterns may be added insulativelyoverlying metal pattern 214. Note here that in the manufacture of thisembodiment shown in FIG. 2, resultant IC structure may not offer thecapability of completely eliminating the flow of tunnel current throughoxide films; in this respect, it might be admitted that this embodimentremains less than the previous embodiment of FIG. 1 in reduction ofpower dissipation. Instead, a significant advantage unique to the FIG. 2embodiment lies in the capability of reducing process complexity andmanufacturing costs. This can be said because the tunnel-current flowdiffering between standard MOS transistors and power supply interceptMOS transistors is attained by merely adding ion doping or implantationprocess steps to the ordinary fabrication system procedure. Regardingafter-manufacture reliability test procedure, the FIG. 1 embodiment ismore advantageous than the FIG. 2 embodiment in that during testroutines the former requires mere execution of measuring the gateinsulation film thickness whereas the latter should require actualoperations of resultant devices manufactured.

A third embodiment will now be described with reference to FIGS. 3 and4. FIGS. 3 and 4 are diagrams each showing one practical configurationof a semiconductor integrated circuit device embodying the invention,wherein FIG. 3 is a layout depiction of this embodiment whereas FIG. 4is a cross-sectional view of the layout taken along line A—A′ of FIG. 3.The IC device is an example having a series combination of two NAND gatecircuits.

In FIG. 3, MOS transistors MP and MN are those for power supplyintercept (for use with a control circuit), and measure 10 nm in gateinsulation film thickness although these remain operable with 5 -nm gateinsulation film thickness. MOS transistors TP and PN are for use with alogic circuit (main circuit), and are 3.5 nm in gate insulation filmthickness. in this way, this embodiment employs two kinds of MOStransistors with different gate insulation film thickness values. Here,the gate length LM of gate insulation film thickness-increased MOStransistors is greater than that of gate insulation filmthickness-decreased MOS transistors. This is based on the fact that theneed is arisen to set an appropriate gate length suitable for the gateinsulation film as mentioned previously; if the gate length remainsshort when such dielectric film is thick, then subthreshold leakage canoccur between the source and drain disenabling complete turn-on/offoperations.

The internal structure of the semiconductor integrated circuit device ofthis embodiment will be explained with reference to FIG. 4. While thisembodiment basically employs thin-film MOS transistors to attainhigh-speed operations, it is further provided with certain switches forintercepting power supply during standby periods of such thin-film MOStransistors in order to minimize power dissipation during standby. And,the switches include thick-film MOS transistors inherently less in flowof tunnel leakage current.

An n-type substrate 301 has therein a p-type well 302 and also has anelement-separation region 303 on substrate 301. Spaced-apart regions 304to 307 are the sources and drains of logic-circuit MOS transistors TPwhereas regions 308 and 309 are the source and drain of a power-supplyintercept MOS transistor MP. MOS transistors TP have insulated gateelectrodes 310, 311; MOS transistor MP has its gate electrode 312.Reference character “GIT” designates the gate insulation film of eachtransistor TP, and “GIM” indicates that of transistor MP.

A first interlayer dielectric film 313 overlying the gates 310-312 onsubstrate 301 has openings as contact holes through which the sourcesand drains as well as gate electrodes of respective transistors areelectrically coupled by first lead layers 314, 315, 316 and 317. Leadlayers 314, 316 are connected to the source regions of logic-circuit MOStransistors pMOSL whereas lead layer 315 is to the “common” drain regionthereof. Lead 317 connects the source of one logic-circuit MOStransistor pMOSL to the drain of power-supply intercept MOS transistorpMOSV. Lead 318 is interconnected to the source of power-supplyintercept MOS transistor PMOSV.

A second interlayer dielectric film 319 formed on the first interlayerdielectric film 318 has a contact hole for use in electricallyconnecting second lead layers 320, 321 to the first lead layer at adesired location thereon. Lead 320 shunts the drain of power-supplyintercept MOS transistor pMOSV. Lead 321 acts as a first power supplyline for shunt of the source of power-supply intercept MOS transistorpMOSV. Lead 321 is interconnected to first lead 318 via the contact holeof second interlayer dielectric film 319. With the above layout,interconnection between a logic circuit formed of logic-circuit MOStransistors PMOSL and nMOSL and the first power supply is controllableby the power-supply intercept MOS transistor pMOSV. Note here thatalthough only the p-type power-supply intercept MOS transistors pMOSVare shown, it also remains permissible to connect n-type power supplyintercept MOS transistors nMOSV each having a thick gate insulation filmbetween logic-circuit MOS transistors nMOSL and a second power supplyline. This configuration will be shown in several circuit diagrams (seeFIG. 5 and FIGS. 6 through 9) to be later presented.

A fourth embodiment of the present invention will be explained withreference to FIG. 5, which is drawn to an inverter circuit of simplestconfiguration.

In FIG. 5 the reference character “L1” indicates a CMOS inverter, “TP1”and “MP1” designate p-type MOS transistors, and “TN1” and “MN1” aren-type MOS transistors. (In this transistor circuit diagram and laterpresented ones of the present application, the small circle symbol “∘”will be adhered to the gate terminal section of each p-type MOStransistor in the illustration.) MOS transistors TP1, TN1 correspond tothose TP, TN of FIG. 1 respectively. The gate insulation films of MOStransistors TP1, TN1 are thinner than those of MOS transistors MP1, MN1.Hereinafter, certain transistors employing a thin gate insulation filmlike MOS transistors TP1, TN1 will be referred to as the “thin-film MOStransistors” or “thin-film transistors”; transistors using a thick gateinsulation film like MOS transistors MP1, MN1 will be called the“thick-film MOS transistors” or “thick-film transistors.” (In thetransistor circuit diagrams of this application, the illustration ofeach thin-film MOS transistor comes with an ellipse surrounding it.)Attention should be taken to the fact that while most prior knownthin-film transistors called the “TFTs” refer to those formed on adielectric substrate by use of semiconductor then-film fabricationtechniques, the thin-film and thick-film transistors according to thisinvention are free from such limitative structure originated from thesemiconductor-on-insulator (SOI) fabrication scheme; importantly,definition of these thin- and thick-film transistors of this inventionis simply based on comparison of the gate insulation film thicknessbetween them.

A thick-film MOS transistor MP1 is inserted between a first power supplyVdd and the CMOS inverter L1 whereas a thick-film MOS transistor MN1 isbetween a second power supply Vss and CMOS inverter L1. Where thiscircuitry is for use in processing signals (during normal operationperiods), a control signal CS is at a logic High or “H” level. Uponreceipt of this control signal, thick-film MOS transistors MP1, MN1 turnon causing first power supply Vdd and second power supply Vss to becoupled directly to CMOS inverter L1. Since this inverter L1 is formedof thick-film MOS transistors TP1, TN1, some leakage current (tunnelcurrent) can Flow between the gate and source as well as between thegate and drain thereof. This leakage current attempts to flow betweenthe first and second power supplies Vdd, Vss via thick-film MOStransistors MP1, MN1 causing power dissipation to increase as a whole.When this circuitry is out of use, namely, during standby periods, thecontrol signal CS potentially drops down at a logic Low or “L” level.When this is done, thick-film MOS transistors MP1, MN1 turn off forcingCMOS inverter L1 to be electrically disconnected or “wrapped” from firstand second power supplies Vdd, vss. The gate-to-source/gate-to-drainleakage current will no longer flow between first and second lowersupplies Vdd, Vss because thick-film MOS transistors MP1, MN1 arerendered nonconductive. In this situation none of first and second powersupply voltages Vdd, Vss are supplied to CMOS inverter L1 rendering itinoperative (its output OUT is in the high impedance state when signalCS is at “L” level) while eliminating an increase in power dissipationbecause of the fact that thick-film MOS transistors MP1, MN1 disenableflow of any leakage current. In this embodiment the thick-film MOStransistors measure 3.5 nm in gate insulation film whereas thin-filmones are 6.0 nm in gate insulation film; however, this invention shouldnot be exclusively limited thereto since the “standby current leakagereduction” effect may be attainable insofar as some difference presentsbetween them in gate insulation film thickness (in other words, wheneverthe tunnel leakage current of thick-film transistors remains less thanthat of thin-film ones). Additionally, circuitry generally called the“clocked-inverter” is typically designed to operate in response to thecontrol signal CS being fed as a clock input, its intended circuitoperation will not be disturbed when transistors MP1, TP1 andtransistors MN1, TN1 are interchanged in connection order as long asthese are in a series connection. The embodiment circuitry isdistinguishable in nature from prior known inverters in that theeffectiveness is lost upon modification of the connection order ofpaired transistors corresponding to those MP1, TP1 as well as alterationof the connection order of transistors corresponding to those MN1, TN1of the embodiment.

Next, a fifth embodiment of the present invention will be explained withreference to FIGS. 6 and 7. This embodiment is three-stage CMOS invertercircuitry employing a series combination of three pairs of thin-filmp-type MOS (PMOS) transistors TP1 to TP3 and thin-film n-type MOS (NMOS)transistors TN1-TN3.

In the drawings PMOS transistors MP1-MP3 and NMOS transistors MN1-MN3are thick-film transistors.

In FIG. 6 this circuitry includes thick-film MOS transistors which areinserted between the first power supply Vdd and respective power supplyelectrodes Vcd1-Vcd3 of three CMOS inverters and also between the secondpower supply Vss and power supply electrodes Vcs1-Vcs3 thereof. Causingthe control signal CS applied upon the thick-film MOS transistors topotentially drop down at “L” level may decrease in magnitude the flow ofgate-to-source/gate-to-drain currents of thin-film MOS transistorsTP1-TP3 and TN1-TN3 reducing power dissipation.

In the embodiment shown in FIG. 7, the sources of thin-film MOStransistors constituting the three stages of inverters are coupled to“virtual” power supply lines Vcd0, Vcs0 with thick-film MOS transistorsconnected between virtual power supply lines Vcd0, Vcs0 and first andsecond power supply lines Vdd, Vss. With such an arrangement, similaradvantages to those in the case of FIG. 6 may be also attained.

Comparing the circuit configurations of FIGS. 6 and 7 with each other,the FIG. 7 configuration will result in a decrease in occupation area ona chip substrate in most cases. It is required that the gate width oftransistors MP1-MP3, MN1-MN3 be carefully determined to eliminateoccurrence of a delay in time during operation of each inverter due toinsertion of transistors MP1-MP3, MN1-MN3. In the case of FIG. 6, thegate width of transistors PM1, MN1 is determined to be identical orequivalent to that of transistors TP1, TN1, by way of example. In thecase of FIG. 7, however, the gate width of transistors MP1, MN1 may bedetermined in view of the activation ratio of each inverter. Morespecifically, the gate width is determined by taking into account themaximum activation ratio of logic circuits (three stages of inverters inFIG. 7) connected to transistors MP1, MN1. In the example of FIG. 7, asingle one of three inverters is rendered operative at a time;accordingly, the gate width of transistors MP1, MN1 is designed atappropriate value insuring sufficient current supply to such singleinverter. This would result in the gate width being the same as those oftransistors MP1-MP3, MN1-MN3 of FIG. 6, rendering the circuitry of FIG.7 smaller in area than the FIG. 6 circuitry.

A sixth embodiment of the invention will be described in connection withFIG. 8. This embodiment is similar to the fifth embodiment shown in FIG.7 with a level holder circuit LH1 being added thereto enablingretainment of the potential level of an output OUT when the controlsignal CS potentially goes low rendering the inverter inoperative sothat it is in the high impedance state. Then control signal CS changesin potential from the “H” to “L” level, the last logic level ismaintained. While this embodiment employs two latch circuits to make upthe level holder LH1, any other configurations may alternatively be usedinsofar as a level holdable circuit employed is capable of holding thepotential level of output OUT when control signal CS is at “L” withoutaffecting its succeeding circuitry of the next stage responsive toreceipt of output OUT.

This embodiment is under an assumption that the level holder LH1 is notrequired to exhibit high-speed operations; thus, thick-film MOStransistors are employed therefor to suppress current leakage. Ifhigh-speed requirement is applied then level holder LH1 may beconstituted by thin-film MOS transistors; in such case; care should bepaid to the circuit design in order to ensure that possible currentleakage therein is not greater than that in the main inverter section.

It should be noted that it is impermissible for the level holder to beunconditionally located anywhere in the circuitry; by way of example,any intended function would not be attainable if it were inserted at a“midway” inverter output OUT1 or OUT2 of the multi-stage CMOS invertercircuitry of FIG. 8. Thus, it is a must for level holder LH1 to beinserted at a selected position associated with a specific signaltransmission line required to hold the logic level even when controlsignal CS is at “L” level—that is, at an output node OUT3 in FIG. 8.

A seventh embodiment of the invention will now be explained by use ofFIG. 9. While in FIG. 5 (fourth embodiment) and FIG. 8 (sixthembodiment) there are shown certain embodiments employing thin-film MOStransistors to form the “inverter,” the principles of the invention mayalso be applied to other types of circuits with any functions insofar asthese are constituted from thin-film MOS transistors. One of suchexamples is shown in FIG. 9. FIG. 9 shows an integrated circuit with theinverter of FIG. 5 being replaced by a NAND gate having two inputs (IN1,IN2). With such an arrangement also, it is possible to prevent anincrease in power dissipation in a manner similar to that of FIG. 5.

In the embodiments shown in FIG. 5 to 9, the control circuit coupled tothe control signal CS employs thick-film MOS transistors havingthick-film oxide films; however, the present invention should notexclusively be limited thereto and may also employ other devices capableof controlling an amount of gate-to-source/gate-to-drain leakage currentof thin-film MOS transistors in response to control signal CS. Use ofMOS transistors greater in gate-electrode depletion ratio than those ofthe main circuit is one example. Using MOS transistors with a specificthin-film gate insulation film of decreased gate leakage is anotherexample.

Further, as per the embodiments shown in FIGS. 5-9, the description isnot specifically directed to how the substrate electrodes of MOStransistors are to be arranged; this is based on the fact that thisinvention is completed regardless of whether such connections areconfigured in practical use. For instance, it may be arranged so thatthe substrate electrodes of p-type MOS transistors are coupled to thefirst power supply Vdd whereas those of n-type MOS transistors are tothe second power supply Vss. Alternatively, in the embodiment of FIG. 5,the substrate electrode of the thin-film MOS transistor TP1 may becoupled to the voltage Vcd1 whereas that of thin-film transistor TN1 isto. Vcs1. In this case and ordinary or standard cell(s) of CMOS inverterwith the substrate electrode being coupled to the power supply may beused without the need of any extra modifications.

The semiconductor integrated circuit device as manufactured by thefabrication schemes described in conjunction with FIGS. 1 and 2 isadaptable for use with any one of all the circuit configurations ofFIGS. 5 through 9. Further, the embodiments of FIGS. 5-9 will offer moresuccessful results if they employ circuitry inherently less in operationfrequency. The term “frequency” as used herein refers to the ratio ofoperative or “active” periods to inoperative or “inactive” periodsthroughout operations thereof. One example is a word-decoder/drivercircuit of memory circuitry. Typically, single-port memory circuitrycomes with multiple word-decoder/driver circuits corresponding in numberto word lines, only one of which is rendered active at a time whilerendering inactive the remaining, increased number ofword-decoder/driver circuits, which in turn results in an increase inpower dissipation due to constant current flow if gate leakage isavailable. Even if this is the case, use of the aforesaid embodiment mayenable power dissipation to decrease in such multiple inactiveword-decoder/driver circuits.

FIGS. 12 to 19 show other examples of the thick-film MOS transistorinsertion method for reducing current leakage during standby periods incircuitry including therein thin-film MOS transistors TP1-TP4 andTN1-TN4.

FIGS. 12 and 13 are examples in the case where an input IN and outputOUT are identical in logic level to each other during standby periods.

As shown in FIG. 12, if it is known that IN=OUT=“H” during standbyperiods then one switch MN1 may be inserted at a node on the Vss sideonly, with no switch used on the Vdd side.

As shown in FIG. 13, if it is true that IN=OUT=“L” during standby thenone switch MP1 may be inserted only at a node on the vdd side with noswitch used on the Vss side. Here, the level hold circuit LH1 isprovided for holding the potential level of an output during standby.

FIGS. 14-17 are examples of the case where the input IN and output OUTare different in logic level from each other during standby periods.

As shown in FIG. 14, where IN and OUT differ in logic level from eachother during standby periods, a switch is inserted at either IN or OUTin order to eliminate occurrence of leakage between IN and OUT. IfIN=“H” and OUT=“L” then insert it at Vss and OUT, or at Vdd and IN. InFIG. 14 a switch MN1 is at Vss while switches MP4, MN4 are at OUT.

In FIG. 15 the switches are inserted not at Vss and OUT but at Vdd andIN (as shown by MP1, MP5, MN5). In cases where a switch(es) is/areinserted at an output node OUT required to offer increased load drivingability or “drivability,” the example of FIG. 15 will be more preferablein practical use because of the necessity of constituting suchswitch(es) by use of a MOS transistor(s) of increased gate width, whichis generally considered undesirable.

As shown in FIG. 16, where IN and OUT differ in logic level from eachother during standby periods, a switch(es) is/are inserted at IN or atOUT in order to prevent current leakage therebetween. Where IN=“L” andOUT=“H,” insert a switch MP1 at Vdd and switches MP4, MN4 at OUT.

In FIG. 17, switches are inserted not at vdd and OUT but at Vss and IN(as shown by MN1, MP5, MN5). Practically, the example of FIG. 17 will bemore preferable because it is generally undesirable to insert switchesat the output node OUT required to offer increased load drivability.

FIG. 18 is an example adaptable where the logic level of nodes IN, OUTis kept unknown during standby periods while IN=OUT is true; in thiscase, switches MP1, MN1 are inserted at Vdd and Vss respectively. Noswitches are required at IN and OUT.

FIG. 19 shows a further example adaptable for use in receiving aplurality of input signals (IN1, IN2). During standby, IN1=“H” andIN2=OUT=“L”; hence, a switch MP1 is at Vdd whereas switches MP5, MN5 areat IN1.

As apparent from the examples of FIGS. 12-19, the insertion location ofthick-film MOS transistors for reduction of gate leakage current ischangeable among practical circuits employed. Consequently, it is notnecessary to limit to exactly the same insertion method throughout theentire circuitry; such switches are insertable at appropriate locationsin a case-by-case manner among various function circuit blocks.

FIGS. 20 and 21 show other examples of the level hold circuit LH1.

Circuitry of FIG. 20 employs a series combination of two inverterstages, wherein transistors of the latter stage are sufficiently less indrivability than those of a logic gate as coupled to an input IN, andyet are significantly greater in tunnel leakage current than the same.

FIG. 21 shows an example with the latter stage of FIG. 20 being modifiedto use a clocked inverter. This may improve the design flexibility as tothe transistor current drivability.

It should be noted that in the above description of the embodiments, noparticular limitations are given with regard to the transistor thresholdvalue; however, it will be recommendable that the thin-film MOStransistors are of low threshold value whereas the thick-film ones arehigher in threshold value than the former. When low threshold-valuetransistors are employed, what is called the “subthreshold” leakagecurrent can flow between the source and drain; even if this is the case,such-source-to-drain subthreshold leakage current may be interrupted orcut off by use of high threshold value thick-film MOS transistors asinserted between the power supplies. Several embodiments as will beshown in FIG. 22 and its following figures of the drawing are drawn tothe circuit configuration basically employing therein a combination ofthick-film MOS transistors of high threshold value—such as 0.5 volts,which may render subthreshold current leakage negligible—and thin-filmMOS transistors of low threshold value such as 0.1 volt, or more orless, by way of example.

It should also be noted that in the above description of theembodiments, although no specific discussions are given in regard to therelation of voltages as input to the gate nodes of thin-film MOStransistors versus input voltages at the gates of thick-film MOStransistors, it will be more effective to design so that the inputvoltages at the thick-film MOS transistor gates are higher than theinput voltages at the thin-film transistor gates. The thick-film MOStransistors are increased in gate insulation film thickness permittingapplication of higher voltages thereto as compared with thin-film MOStransistors whereby current drivability of thick-film MOS transistorsmay increase accordingly. In the embodiments of FIGS. 5 through 21, thiscan be attained by increasing the amplitude of signals CS and /CS. Insuch case the thick-film MOS transistors are to be greater in gatelength than the thin-film MOS transistors. This serves to increase thethreshold value of thick-film MOS transistors while simultaneouslyenhancing device reliability of high-voltage operable thick-film MOStransistors. In some embodiments of FIG. 22 and its succeeding figuresof the drawing, there will be shown circuit configurations basicallyarranged to apply a high voltage of 3.3 volts to thick-film MOStransistors while applying a low voltage of 1.5 volts to thin-film MOStransistors, by way of example.

Several types of exemplary semiconductor integrated circuit devicesemploying the principles of the present invention will be describedhereafter.

See FIG. 22, which is a block diagram of a semiconductor integratedcircuit device embodying the invention. In the following figures of thedrawing, different kinds of lines are used for clarity purposes: Solidlines are used to indicate circuit blocks mostly including thin-film MOStransistors in light of the area ratio; broken or dotted lines are tocircuit blocks mainly employing thick-film MOS transistors; and,solid-and-dotted line pairs are to circuit blocks each using boththin-film and thick-film transistors therein.

A main circuit 2201 including a CPU core receives input signals andissues output signals via an input/output (I/O) circuit 2202. The maincircuit 2201 also accesses a memory cell array 2205 (DRAM, for example)via a memory-direct peripheral circuit 2204 to receive from and transmitsignals to it. A standby control circuit (power-supply control circuit)2206 is provided for control of selective feed of a power supplyvoltage(s) to thin-film MOS transistors within respective modulesmentioned above. Typically, internal signals of semiconductor integratedcircuit devices are different in amplitude from those outside them. Tocompensate for such signal amplitude difference, a level convertercircuit to be later described is provided for conversion of potentiallevel therebetween.

In FIG. 22 the memory cell array 2205 is configured using selected kindof MOS transistors negligible in amount of tunnel leakage current(thick-film MOS transistors). The gate insulation film of suchtransistors may be an oxide film which is as thick as 5 to 10 nm, by wayof example.

The main circuit 2201, I/O circuit 2202, memory-direct peripheralcircuit 2204 and standby controller 2206 employ as their main elementsthin-film MOS transistors. Particularly, the main circuit containingtherein logic elements is increased in ratio of thin-film MOStransistors used.

As has been fully discussed in connection with FIGS. 5-21, the thin-filmMOS transistors in these circuits are arranged such that they arecapable of interrupting or “intercept” the power supply by switches inorder to reduce current leakage during standby periods. Thick-film MOStransistors are used for such power-supply intercept switchesdisenabling flow of leakage current through these switches per se. Thesepower supply switch MOS transistors turn on and off selectively undercontrol of the standby controller 2206.

This semiconductor integrated circuit device is arranged to employthick-film MOS transistors also for certain transistors (such as thosein I/O circuit) which directly receive an input of relatively largesignal amplitude from the exterior of the IC chip, other than thethick-film MOS transistors for use with the power supply switches. Thisis because of the fact that higher gate-withstanding voltage MOStransistors are required for the I/O circuit to which significantamplitude of signals are input, and that thick-film MOS transistors aretypically greater in gate voltage breakdown level. The thick-film MOStransistors for gate leakage reduction in the thin-film MOS transistorsas discussed previously with reference to FIGS. 5-21 may be employed asthe high voltage MOS transistors for use with the I/O circuit. It ispossible to reduce process complexity by using the thick-film MOStransistors for the both MOS transistors.

The memory cell array 2205 includes rows and columns of memory cells asrequired to continue storing data during standby periods, which areformed of thick-film MOS transistors negligible in tunnel leakagecurrent. Employing thick-film MOS transistors for these memory cellsmight cause the operation speed to decrease; on the other hand, thisenables resultant circuit to be free from the power dissipation increaseproblem due to gate current leakage, making it possible to continuouslytransfer power supply voltages to the memory cells even during standby.Conversely, thin-film MOS transistors are used for certain memory cellsthat are not required to retain information therein during standbyperiods. During standby, information stored in memory cells willdisappear; however, it is possible by interrupting transfer of the powersupply voltage to the memory to eliminate an increase in powerdissipation due to gate leakage. Also, in cases where the memory is lessin data storage capacity so that continuous power feed results in a merenegligible increase in gate-leakage power dissipation, the memory cellmay similarly be formed of thin-film MOS transistors. For example,register files are inherently small in capacity rendering resultantleakage current negligible in the practical sense; rather, these aremore important in operation speed. Desirably, such memory is formed ofthin-film MOS transistors. In the semiconductor integrated circuitdevice of this embodiment, it is preferable that certain memory circuitssuch as latches, flipflops, and the like employ thin-film MOStransistors. On the other hand, high-voltage/low-speed circuits whichare driven with high voltages and are not required to attain rapidresponsibility, such as the power supply control switches stated supra,for example, are preferably formed of thick-film MOS transistors.

In the example of FIG. 22, the IC chip is driven using at least twokinds of power supply voltages Vcc1, Vcc2, where Vcc2 is higher thanVcc1. The thick-film MOS transistors are to be driven by power supplyvoltage Vcc2 of increased current supplying ability whereas thethin-film MOS transistors are by Vcc1. It can be readily seen by thoseskilled in the semiconductor art that while the following embodimentsassumes use of Vcc1 of 1.5 volts and Vcc2 of 3.3 volts, such potentialvalues may freely be modified insofar as they satisfy the relation ofVcc2 >Vcc1.

With the semiconductor integrated circuit device of FIG. 22, high speedoperation is expectable since thin-film MOS transistors are used formost of the major units therein.

A semiconductor integrated circuit device also embodying the inventionis shown in FIG. 23. This device is basically formed of a main circuit2301 including logic circuits and the like, I/O circuit 2302, andstandby control circuit 2303. In this example the voltage Vcc2 of 3.3volts as externally supplied thereto is passed to a potential reductioncircuit 2304, which causes voltage Vcc2 to drop down at 1.5 volts toprovide internal power supply voltage Vcc1. Potential reduction circuit2304 may be on the same chip substrate together with the main circuitand others; or alternatively, it may be formed on a separate chip. Maincircuit 2301 is mostly constituted from thin-film MOS transistors tospeed up its operation. Potential reduction circuit 2304 mainly consistsof thick-film MOS transistors. I/O 2302 and standby controller 2303include a combination or “hybrid” of thin-film and thick-film MOStransistors therein. In these circuits the thin-film MOS transistors aredriven with Vcc1 whereas thick-film ones are with Vcc2. Standbycontroller 2303 operates to turn off an output of potential reductioncircuit 2304 during standby periods in order to reduce power dissipationdue to current leakage. Controller 2303 also causes an output from I/O2302 toward main circuit 2301 to change at “L” level. When an input tomain circuit 2301 is at “L” while the power supply voltage fed theretois at zero volts, then most nodes within main circuit 2301 are at “L”reducing power dissipation due to tunnel-current leakage. As can bereadily seen by those skilled in the art, where the thin-film MOStransistors are low in threshold value, resultant power dissipation dueto flow of subthreshold leakage current will be reduced accordingly.

A further embodiment is shown in FIG. 24, wherein like referencenumerals are used to designate like parts in the embodiment of FIG. 23.In this embodiment two kinds of power supplies Vcc1, Vcc2 are externallyfed to the IC chip; Vcc1 is supplied to main circuit 2301 and others viaa switch 2404 as formed of a thick-film PMOS transistor. During standbyperiods, standby controller 2303 causes switch 2404 to turn offinterrupting feed of power supply Vcc1. Like the embodiment of FIG. 23,the output of I/O 2302 to main circuit 2301 is forced to change at “L”level. Switch 2404 may be on the same chip along with main circuits 2301and others, or alternatively may be a discrete power MOS transistorexternally wired to the chip. Here, switch 2404 is a thick-film MOStransistor. In a similar way to that of the FIG. 23 embodiment, when aninput to main circuit 2301 potentially changes at “L” while the powersupply fed thereto is at 0 volts, internal major nodes of main circuit2301 are at “L” reducing power dissipation due to tunnel currentleakage.

A semiconductor integrated circuit device embodying the invention isshown in FIG. 25, which is similar to that of FIG. 23 with a specificcircuit being built therein for compensating for any possible variationin operation speed of the main circuit. In this drawing also, like partshave the same reference numerals. In this embodiment the main circuit2501 comes with a delay monitor circuit MON1. Delay monitor circuit MON1is for monitoring in principle a delay time of logic circuits withinmain circuit 2501. Accordingly, delay monitor circuit MON1 is formedusing thin-film MOS transistors which are the same as those of maincircuit 2501. Delay monitor circuit MON1 may be a ring oscillator, byway of example. A potential reduction circuit 2504 is provided forreceiving voltage Vcc2 to issue a potentially decreased voltage Vcc1,and is responsive to receipt of a signal from delay monitor circuit MON1in main circuit 2501, for controlling the value of voltage Vcc1 in sucha way as to compensate for deviations of delay time of logic circuitswhich constitute main circuit 2501 due to environmental variations suchas a variation in process parameter during the manufacture ofmain-circuit transistors and/or in ambient temperature. This may beattainable by use of phase-locked loop (PLL) schemes one of which willbe later described in connection with FIG. 28. By way of example,suppose that the temperature rises increasing the delay time of logiccircuits constituting main circuit 2501. If this is the case, potentialreduction circuit 2504 attempts to potentially increase its outputvoltage Vcc1. Conversely, if temperature drops decreasing the delay timeof the logic circuits of main circuit 2501, then potential reductioncircuit 2504 forces output Vcc1 to decrease in potential. Whereby, thelogic circuits forming main circuit 2501 may be kept constant in delaytime throughout operation.

A semiconductor integrated circuit device further embodying theinvention is shown in FIG. 26. While in FIG. 25 the monitor circuit MON1is provided to monitor delay time of the logic circuits constituting themain circuit, this embodiment is arranged so that the characteristics ofMOS transistors or logic circuits which constitute the main circuit aremeasured during the reliability test procedure in the manufacture of ICchips while allowing resultant device information to be stored in amemory unit 2605. Upon receipt of a control signal issued from this“device info” memory 2605, a potential reduction circuit 2604 operatesto determine an exact value of voltage Vcc1. By way of example, imaginethat the transistors constituting main circuit 2301 under manufactureare greater in threshold value than expected by circuit design. If thisis the case, store such data in memory 2605 so as to cause potentialreduction circuit 2604 to generate and issue a modified or “updated”value of voltage Vcc1 which is greater than the initially designedvalue. Alternatively, where the chic reliability test result reveals thefact that the threshold value of the transistors forming the maincircuit, store corresponding data in memory 2605 causing potentialreduction circuit 2604 to generate and issue another updated value ofvoltage Vcc1 which is below the initial design value. With such a schemeemployed, is possible to compensate for fabrication deviations.Additionally, the device information to be stored in memory 2605 may betransistor threshold value, transistor saturation current value, or anyother equivalent parameters thereof which reflect the delay time of thelogic circuits constituting the main circuit. Also, as for the storagemethod thereof, any scheme may be employed. One recommendable simplestorage scheme is as follows: a method of changing the value of areference voltage Vref of a potential reduction circuit shown in FIG. 27by physical methods using the FIB processing for cutting away a fuse(aluminum wiring lead) by an ion beam.

While the method of FIG. 25 may compensate for environmental variationssuch as the process parameters in the manufacture of main-circuittransistors, ambient temperatures and others, the method of FIG. 26 mayoffer capability of compensating for the process parameters in themanufacture or main-circuit transistors only. Instead, the latter ismore advantageous than the former in that any area overheads can besuppressed or minimized with simple architecture used.

Other approaches other than those of FIGS. 25 and 26 may be stillavailable for compensating for environmental variations such as theprocess parameters in the manufacture of main-circuit transistors,ambient temperatures and others, which approaches are also within thescope of the present invention.

One exemplary circuit configuration of the potential reduction circuit(voltage limiter) 2304 of FIG. 23 for conversion of high voltage Vcc2 tolow voltage Vcc1 is shown in FIG. 27. This voltage limiter 2304 iscontrolled in responding to a control signal fed from the standbycontroller 2303, for turning on and off transfer of voltage Vcc1. Thisvoltage limiter handles relatively high voltages, and therefore isprincipally formed of thick-film MOS transistors. Note however that itremains permissible for a phase compensation capacitor CC to exhibitcurrent leakage which is as less as several microamperes (μA). Employingthin-film MOS transistors enables resultant circuitry to decrease inchip area. In particular, capacitor CC typically ranges from severalhundreds to thousands of picofarads (pF) in capacitance, which in turnserves advantageously to reduce the chip area. Transistors forming avoltage divider circuit DIV1 are also allowed to exhibit flow of minuteleakage current on the order of several μA; even when such leakagearises, MOS transistors each having a thick gate insulation film may beemployed since this circuit will merely act as a voltage dividingresistor.

FIG. 28 depicts a detailed configuration of the delay monitor circuitMON1 and potential reduction circuit 2504 shown in FIG. 25. Delaymonitor circuit MON1 employs a ring oscillator including a chain of CMOSinverters. This circuit defines PLL circuitry. A frequency-phasecomparator PFD is connected for comparing the oscillation frequency ofdelay monitor circuit MON1 with a clock signal f1 fed to the maincircuit, and for driving an associated charge-pump circuit CP throughlevel converters LC3. An output of charge pump CP is sent forth througha low-pass filter LPF to be issued at an output as the reference voltageVref, based on which the voltage Vcc1 will be produced in conformitywith a clock signal f1. Here, ring oscillator MON1 and comparator PFDare formed of thin-film MOS transistors. Charge pump CP employsthick-film MOS transistors since it uses voltage Vcc2 as its powersupply. Rendering the main circuit operative in synchronism with clocksignal f1 may enable it to operate with an appropriate power supplyvoltage as maximally optimized to the clock frequency thereof.

FIG. 29 shows another exemplary configuration of the delay monitor andpotential reduction circuits of FIG. 28. The circuitry of FIG. 29 issimilar in basic configuration to that of FIG. 28 with the power supplyVcc1 fed to the main circuit being separated from a power supply Vcc3 asfed to the delay monitor circuit. Voltages Vcc1, Vcc3 are inherentlyidentical in potential to each other; however, Vcc1 can experiencemixture of noises from the main circuit. In this respect, in order tosuppress adverse influence of the Vcc1 noises upon the delay monitorcircuit, the power supply Vcc3 for the delay monitor circuit isindependent from Vcc1 thus improving the monitoring accuracy.

FIG. 30 illustrates an exemplary configuration of the I/Os 2202, 2302 ofFIGS. 22-26. This drawing shows one-bit part only. The I/O handlesin-chip signals and external signals via an I/O terminal pad PAD. WhenSEL is at “L” level, PAD acts as an input terminal; when SEL is at “H,”PAD acts as an output terminal. A level converter LC1 is responsive toreceipt of a control signal STB issued from standby controllers 2206,2303 (see FIGS. 22-26); when STB is at “L” then converter LC1 converts asignal with the amplitude of Vcc1 into a signal of an increasedamplitude of Vcc2, which is sent forth from output terminal PAD.Accordingly, certain thick-film MOS transistors connected between levelconverter LC1 an I/O terminal PAD are formed of thick-film MOStransistors. Here, a signal PULL is input to the gate of a pull-up PMOStransistor in such a way that when pullup is required, the signalpotentially falls at “L” level permitting execution of pullup operationby such PMOS transistor. This PMOS transistor is a thick-filmtransistor. During standby periods of the IC chip shown, the standbycontrol signal STB potentially rises at “H” causing level converter LC1to hold or retain the last potential level of an output since theon-chip thin-film MOS transistors are prevented from receiving any powersupply voltages due to power supply interception.

On the input side,.an inverter consisting of a pair of MOS transistors4004P, 4004N receive an externally supplied input signal with amplitudeof Vcc2 for conversion to a signal having the amplitude equivalent toVcc1. Accordingly, these two transistors handling such level-convertedsignal are formed of thick-film MOS transistors. During standby periodsany signal from I/O terminal PAD is cut off by a PMOS transistor 4015P2forcing an input signal IN to be potentially fixed at “L.”

Resistors R1-R2, diodes 4002D1-4002D2, transistor 4014N are connectedforming an input protector circuit. Additionally, diodes 4002D1, 4002D2may be formed of MOS transistors. Those MOS transistors included in thisinput protector are high-voltage thick-film MOS transistors.

FIG. 31 shows one practical configuration of the level hold and levelconverter circuit LC1 of FIG. 30. Level holder circuit 3101 isresponsive to standby control signal STB for potentially holding asignal of Vcc1 amplitude; thereafter, the resulting signal is convertedby level converter 3102 into a signal of Vcc2 amplitude which is thengenerated at output OUT.

FIG. 32 shows another practical configuration of the level hold andlevel converter circuit LC1 of FIG. 30. In this embodiment the standbycontrol signal STB is supplied at a midway node between a level holder3201 and level converter 3202. Level holder 3201 is placed at a locationnear the output side as looked at from converter 3202, for potentiallyholding a converted signal with the Vcc2 amplitude.

Upon comparing the embodiments of FIGS. 31 and 32, it may be appreciatedby experts in the art that these are functionally identical to eachother in performing level conversion of a small amplitude (Vcc1) signalinto an increased amplitude (Vcc2) signal and in continuing output ofthe last potential value after signal STB changes at “H” level. Notehere that the former is more advantageous then the latter in thatnecessary chip area remains less.

FIG. 33 shows one exemplary configuration of the standby controllers2206, 2303 (see FIGS. 22-26). The circuitry shown receives an inputsignal STBIN (Vcc1 amplitude) from the main circuits 2202, 2301 or thelike and generates by level conversion an inverted output signal/STB.This circuitry is not under strict high-speed requirements, and thus ismostly formed of thick-film MOS transistors in order to suppress flow ofleakage current, except that certain portions of it handing signals ofVcc1 amplitude employ thin-film MOS transistors (as enclosed by ellipsetemplates to distinguish them from the others). In light of the factthat an associated circuit for issuing the STBIN signal can also be inthe wait or standby state in response to the STB signal, the circuitryof FIG. 33 prevents the STBIN signal from becoming unstable duringstandby periods by causing transistors 3301, 3302 to latch the STBoutput for retainment of its potential level.

A microcomputer system embodying the invention is shown in FIG. 34. Dueto significance of data storage capacity that accompanies with the gateleakage problem, a command/instruction cache register 3402 and memorycells 3403, 3404 employ thick-film MOS transistors. If attaininghigh-speed characteristics is more important than power dissipationreduction then certain elements under such high speed requirements areformed of thin-film MOS transistors providing a hierarchical memorystructure. Likewise, TLBs (included in blocks 3410, 3411) and registerfiles (in blocks 3405, 3406) are mostly comprised of thin-film MOStransistors reducing power dissipation therein.

The microprocessor of FIG. 34 also includes instruction issuance unit3412, general-purpose register 3405, floating point register 3405,integer arithmetic unit 3407, floating point arithmetic unit 3408, andload/store unit 3409, which are formed of thin-film MOS transistors inview of the fact that they are under high-speed requirements butnegligible in affection of power dissipation as long as capacity remainssmaller. Standby controller 3413 and I/O 3414 may be similar to thosediscussed supra.

FIG. 35 shows a cross-sectional device structure of the I/O unit shownin FIG. 30. In this drawing the part designated by reference character“A” is an input protector circuit whereas part “B” is an I/O circuit,which contains therein a level converter circuit.

An n-type silicon substrate 4006 has a p-type well 4007P and n-type well4007N with an element separation region 4008 being provided thereon.Impurity-doped layers 4010P1, 4009P1, 4009P, 4010N1 are provided as thesource and drain regions of an input protection MOS transistor pMOSL. R1and R2 are resistive elements; 4004P1, 4004P2, 4000N2, 4000P2, 4004N1and 4004N2 are the source and drain regions of level conversion MOStransistors. 4004N4, 4004P4 are the gate electrodes of MOS transistors4004P, 4004N. 4004N3, 4004P3 are the gate insulation films of MOStransistors 4004P, 4004N. A wire lead 4013 is for supplying the powersupply through contact hole 4012. Here, the level converter in the I/Ocircuit area employs thick-film MOS transistors 4004P3, 4004N3. Theremaining arrangement is similar to that shown in FIG. 4.

FIGS. 36 through 42 show some exemplary configurations of a mask ROM.Typically, mask ROMs are designed to store binary data either by causingelectrical charge as precharged on a bit line to discharge at groundpotential or by forcing such precharged charge to be held thereon at aspecified high potential.

FIG. 36 is a functional block diagram of a NOR type mask ROM whichemploys the diffusion-layer programming scheme. A word decoder 3604 isconnected to receive N sets of row addresses and M sets of columnaddresses (where N and M are predefined integers) for selection of asingle address (with one memory cell as a unit). A word driver 3602 isresponsive to an output of word decoder 3604 for driving the memory cellwith selected address. Note that since memory cells here are formed ofthick-film MOS transistors as will be later described, the word lineamplitude is set at a higher potential (Vcc2). Accordingly, word driver3602 employs thick-film MOS transistors applied with the voltage of anincreased amplitude while causing address signals of decreased potentialamplitude (Vcc1) as normally fed from an associative CPU to be coupledto word driver 3602 after conversion of such Vcc1 signals into Vcc2signals at the level converter. Word decoder 3604 handlingsmall-amplitude signals employs thin-film MOS transistors. A memory cellarray 3601 includes a predefined number of rows and columns of memorycells employing thick-film MOS transistors because of the fact that ifotherwise these cells were formed of thin-film transistors then unwantedleakage current could flow via associated word lines, where a maximalamount of such leakage current may be defined by the number of cellsoperatively associated with one bit line multiplied by gate leakagecurrent per cell. Use of thick-film transistors for cells allows thememory cells associated with on bit line to increase in number, which inturn permits enhancement of advantages of the invention with an increasein data storage capacity of memory array 3601. If memory array 3601 werearranged using thin-film transistors then non-selected cells could causeflow of leakage current adversely behaving to impose noise mixture onbit lines, which results in a decrease in signal-to-noise (S/N) ratiowhile increasing the risk of occurrence of malfunction. A levelconverter 3603, sense amplifier 3605 and standby controller 3606 maycontain both thin-film and thick-film MOS transistors.

In the mask ROM of FIG. 36, certain memory cell MMN00 for storage of alogic “1” data bit comes with no transistors. In other words, the “1”storage cell does not have any diffusion layer. When one word line W12goes high (“H”) in potential, its associated cell MMN11 turns on causingbit line BL1 to drop down at “L” level. In this situation the data “0”storage cell MMN11 constitutes a transistor which will not go low evenwhen word line W11 goes high.

FIG. 37 shows a NOR mask-ROM employing the ion implantation programmingscheme. This mask-ROM is similar to that of FIG. 36 with the memory cellarray being slightly modified in configuration. On occasions where wordlines W21, W22 potentially go high, the turn-on/off control of celltransistors is controlled based on the threshold value Vth of each MOStransistor.

FIG. 38 is a cross-sectional view of one memory cell of FIG. 37. Thelogic level of stored data is determined by verifying whether acorresponding MOS transistors turns on or off when its associated wordline is selected. The word line is equivalent in potential to an outputsignal of word driver 3602; hence, it is Vcc2 (>Vcc1) in this case.Accordingly, the terminology “high threshold value Vth” refers toVth>Vcc2. In the case of low Vth, the relation Vth<Vcc2 is establishedsince it is sufficient for MOS transistor to turn on. In this embodimenta diffusion layer for reduction of Vth is provided beneath the gateinsulation film of a MOS transistor coupled to word line W21.

FIG. 39 shows a NOR mask-ROM employing the contact-hole programmingscheme. This ROM is similar to that of FIG. 36 with the memory cellarray being slightly modified in configuration. MOS transistors MMN31,MMN32 are of similar arrangement but of different operation such thateach controls the “H” and “L” of an output depending upon whether it isconnected to bit line BL3.

FIG. 40 shows a cross-section of major part of one memory cell of thearray of FIG. 39. As shown, the right-hand side MOS transistor is notconnected to bit line BL3.

FIG. 41 shows a SAND mask-ROM employing the ion implantation programmingscheme. This ROM is similar to that of FIG. 36 with its memory cellarray 4101 being slightly modified in configuration. MOS transistorsconstitute a cell block. A storage data bit is definable in logicvalue—“1” or “0”—depending upon whether these MOS transistors are of thepositive polarity in threshold value (enhancement type) or of negativepolarity (deletion type). In this embodiment a MOS transistor MMN4 n isof the depletion type. When one selected word line BS4 potentially goeshigh, a block-select transistor BSMN4 turn on. Simultaneously, any oneof the word lines associated with this block is selected to go low inpotential. Suppose a word line W4 n is selected. In this block cell acurrent rushes to flow allowing the “L” level signal to be output viablock-select transistor BSMN4.

FIG. 42 illustrates a cross-section of main part of the memory cell ofFIG. 41. Like elements have the same reference characters.

While the principles of the invention are applicable to several types ofmask-ROMs as discussed above, it will be readily seen by experts thatthe current leakage reduction effect of the invention will be maximizedwhen applied to NOR type memory devices which inherently tend toexperience an increased amount of leakage current due to their structurein which input nodes are increased in number resulting from use of agreat number of parallel arrays of MOS transistors therein.

A DRAM device also embodying the invention is shown in FIG. 43. ThisDRAM includes an I/O circuit 4311, standby controller 4306 and worddriver 4312, most portions of which are formed of thick-film MOStransistors and which are designed to operate with power supply voltagesVcc2, Vpp that are higher than Vcc1. A respective one of transistorswithin a memory array 4301 employs a thick-film MOS transistor in orderto prevent electrical charge from leaking out of an associated datastorage capacitor. For drive of memory-cell thick-film transistors, wordlines W are arranged to carry and handle large amplitude voltagesignals. In this case, for purposes of elimination of current leakagefrom capacitors otherwise occurrable in prior known DRAMs, it isrecommendable that memory cell transistors be of high threshold voltage.Decoders 4313, 4318 and address buffers 4315-4316 handling smallamplitude signals are formed of thin-film MOS transistors for drive bythe low voltage Vcc1, supra. A sense amplifier 4305 consists of acombination of thick-film and thin-film MOS transistors.

Input circuit 4311 receives at its input an address signal Ai which isas great as Vcc2 in amplitude. This input signal is level-converted intoa small Vcc1 amplitude signal, which is then passed to address buffers4315-4316 and decoders 4313, 4318. In view of this, the input circuitpreferably employs thick-film MOS transistors at its certain part in theprestage of such level conversion to Vcc1. For the same reason an outputcircuit 4320 employs thick-film MOS transistors. The DRAM shown issimilar to the previous embodiments in that thick-film MOS transistorsare for use in controlling the power supply(ies) as fed to the thin-filmMOS transistors in address buffers 4315-4316 and decoders 4313, 4318.Although not visible in FIG. 43, row decoder 4313 contains therein alevel converter for converting the Vcc1 amplitude signal into anamplitude-increased signal (Vpp) which is then supplied to word driver4321.

In this embodiment the voltage Vcc2 is set at 3.3 volts; Vcc1 is 1.8volts; Vpp is 3.6 volts; and, VDD is 1.5 volts. These voltages may beexternally given or alternatively be internally prepared using anon-chip chip voltage converter.

FIG. 44 depicts an inside configuration of the sense amplifier 4305 ofFIG. 43. This sense amplifier includes a pair of parallel bit lines B,/B, which are precharged at VDD1/2 by a precharge circuit PC duringstandby periods. Sense-amplifier drive lines NCS, PCS are at VDD1/2level. Accordingly, the sources, drains and gates of transistors TP11,TP12, TN11, TN12 constituting one sense amplifier unit SA are all at thesame potential level so that neither subthreshold leakage current nortunnel leakage current flows therein. Thus, these are formed ofthin-film MOS transistors enabling sense operations to increase inspeed.

A precharge signal PCB is at high potential (>VDD1/2) during standby.Hence, transistors MN11-MN13 constituting the precharge circuit arespecific MOS transistors each having a thick gate insulation filmeliminating tunnel current leakage. These are not required to be of thehigh threshold value since the source and drain of each transistor iskept identical in potential.

Transistors MN14-MN15 forming an I/O gate YG are also thick-film MOStransistors. This is in view of the fact that these transistors receiveat their gates an output YS of an associative column decoder and are setat ground potential during standby periods.

FIG. 45 shows a detailed configuration of the sense amplifier circuit4305. Input signals thereto involve address signals Ai, Aj and a timingsignal φ. In currently available standard memory devices, since thememory array is subdivided into portions called the “subarrays,” certainaddress signal (normally, several upper bits of those of a row addresssignal used) is required to render operative only sense amplifier unitsassociated with a selected subarray. A NAND gate NA1 and invertersIV1-IV2 employ thin-film MOS transistors. During standby, the Ai, Aj, φsignals are at “L” level and signal SAN is also at “L” while SAP is at“H”; accordingly, switches are inserted between the Vcc1 line and thepower supply nodes of NAND gate NA1 and inverter IV2 as well as betweenthe ground node of inverter IV2 and ground, for selective interruptionof power feed during standby in order to eliminate tunnel currentleakage. Level hold circuits LH1, LH2 are provided for maintaining thepotential levels of signals SAN, SAP.

Precharge circuits NCS, PCS employ thick-film MOS transistors. Drivetransistors MN20, MP20 make use of thick-film MOS transistors. This isto prevent flow of leakage current between the sources, drains and gatesof these transistors which are at different potential levels duringstandby.

FIG. 46 shows one exemplary configuration of the main amplifier 4309 ofFIG. 43. As shown, this main amplifier consists of a combination of twodifferential amplifier stages MA1, MA2, which employ thin-film MOStransistors to speed up amplification operation. This is in light of thefact that no current leakage can occur because a precharge circuit (notshown) forces amplifier input signals DO, /DO and first stage outputsignals D1, /D1 moreover second stage output signals D2, /D2 to be keptat “H” level during standby periods. On the contrary, activationtransistors MN31-32 are formed of thick-film MOS transistors preventingcurrent from leaking into a node coupled to voltage VSS.

A SRAM further embodying the invention is shown in FIG. 47. The SRAMshown is similar in circuit configuration to the ROM and DRAM devices asdiscussed previously except that the structure of its memory array 4701is distinguishable thereover. Memory array 4701 basically employstherein flipflop circuits which are formed of thick-film MOStransistors.

Of those transistors constituting memory array 4701, at least datatransfer transistors (also called the “access transistors”) are to beformed of thick-film transistors because of the fact that if otherwisethese were formed of thin-film transistors then unwanted leakage currentcould flow via associated word lines into bit lines, where a maximalamount of such leakage current may be defined by the number of cellscoupled to one bit line multiplied by gate leakage current per cell,thereby causing flow of leakage currents adversely behaving to inducenoise mixture on bit lines resulting in a decrease in S/N ratio. Gateleakage current of the remaining transistors other than such transfertransistors is devoted to an increase in power dissipation only;therefore, unless power dissipation is so important, these may bethin-film MOS transistors. The greater the number of memory cellsconnected to one bit line, i.e. the data storage capacity of memory, thegreater the significance of the advantages of this invention.

The same goes with the transfer-transistor threshold value also. If thetransfer transistors were less in threshold magnitude then undesiredleakage current could flow into bit lines, where a maximal amount ofsuch leakage current may be given by the number of cells coupled to onebit line multiplied by source-to-drain subthreshold leakage current oftransfer transistor per cell. This adversely serves to induce noises onbit lines causing S/N ratio to decrease. To eliminate. this, thetransfer transistors are increased in threshold value. This isattainable by adequate adjustment of an amount of impurity as implantedinto the channels of such transfer transistors; or alternatively, thesame may be attained by designing them so that the gate length issomewhat increased.

FIG. 48 shows an exemplary configuration of the word decoder 4704, worddriver 4702 and level converter 4703. Word decoder 4704 receives at itsinput small-amplitude signals, and thus is formed of thick-film MOStransistors while further including a thick-film MOS transistor MN11 forinterruption of gate leakage current during standby. Word linesincluding one WL of FIG. 48 are driven with large-amplitude signals sothat word driver 4702 is designed to operate with the power supplyvoltage Vcc2 with a level converter 4703 being inserted between worddecoder 4704 and driver 4702. Level converter 4703 is for potentiallevel conversion from a small-amplitude to a large-amplitude signal, andthus employs thick-film transistors at its main part. This is generallysimilar to that discussed previously in connection with FIG. 33.

The standby control signal STB goes high during standby periodsrendering power supply Vcc1 off. Thick-film MOS transistor MN12 forcesan output WL2 of level converter 4703 to go high (at 3.3 volts) causingword line WL to be potentially held at “L” level (zero volts). This mayprevent current from leaking into a bit line(s) out of a memory cell(s)during standby.

The word decoder 4704, word driver 4702 and level converter 4703 may bebasically similar in configuration to those used in the SRAM and DRAMdevices stated supra.

FIG. 49 shows a detailed configuration for practical use of a senseamplifier/write circuit 4705 of FIG. 43. The bit-line potential does notaffect data storage so that power supply Vcc1 may be off during standby.This sense amplifier/write circuit employs thin-film MOS transistors.

Industrial Useability

With the foregoing semiconductor integrated circuit devices inaccordance with the present invention, significant practical advantagesare attainable with regard to the fact that it is possible to reducepower dissipation during standby periods without having to slow thecircuit operation.

What is claimed is:
 1. A method of manufacturing a semiconductor devicecomprising on the same silicon substrate a plurality of field-effecttransistors each having a lamination of a gate insulation film and agate electrode, said plurality of field-effect transistors including afirst field-effect transistor and a second field-effect transistorwherein a thickness of the gate insulation film of the firstfield-effect transistor is larger than a thickness of the gateinsulation film of the second field-effect transistor, said methodcomprising the steps of: forming a first layer and a second layer forthe formation of the gate insulation film and the gate electrode of saidfirst field-effect transistor; patterning the first layer and the secondlayer to form the gate insulation film and the gate electrode of thefirst field-effect transistor and removing the first layer and thesecond layer without removing the formed gate insulation film and thegate electrode of said first field-effect transistor; forming a thirdlayer and a fourth layer for the formation of the gate insulation filmand the gate electrode of said second field-effect transistor; andpatterning the third layer and the fourth layer to form the gateinsulation film and the gate electrode of said second field-effecttransistor.
 2. A method according to claim 1, wherein the thickness ofthe gate insulation film of the first field-effect transistor is 10 nmand the thickness of the gate insulation film of the second field-effecttransistor is 3.5 nm.
 3. A method according to claim 1, wherein thefirst layer and the third layer is a thermal oxide layer.
 4. A methodaccording to claim 3, wherein dielectric films are formed on theopposite side walls of the gate electrode of the first field-effecttransistor.
 5. A method according to claim 4, wherein dielectric filmsare made of silicon nitride.